A time delay is commonly imposed during radio frequency (“RF”) signal transmission to provide for proper matching to the control paths. For example, feedforward linear power amplifiers (LPAs) employing high-power low-loss time-delay elements are used to provide time delay matching between the main and error feedforward paths. As a result, the amount of error correction is maximized over the widest possible operating bandwidth. This type of delay function is typically provided by an aluminum-block comb-line bandpass filter or a low-loss coaxial cable. Both common solutions, however, contain various benefits and inherent problems.
Coaxial cables, sometimes referred to as delay lines, were first used in the industry to obtain such an RF delay. They are currently still in widespread use. The benefit of coaxial cables is that there is no special tuning required. Thus, they are simple to use. Unfortunately, they tend to be bulky and expensive. For example, a longer coaxial cable may have to be installed to ensure a longer RF delay even though such a long bulky cable is not otherwise needed for the connection. Moreover, depending upon the quality and length of the coaxial cable, it can cost more than $100 dollars per installation, which can be quite costly when multiplied by hundreds of installations. As a result, other solutions, such as the aluminum-block comb-line bandpass filter, are often used in place of coaxial cable.
A typical aluminum-block comb-line bandpass filter is shown in FIG. 1 and indicated generally at 10. As shown, an input port 12 and an output port 14 are placed on a bottom surface 16 of the filter 10. On a top surface 18 of the filter 10, two screw holes 20, 22 are provided for mounting of the filter 10. A tuner 24 is sandwiched between the top surface 18 and the bottom surface 16, which provides multiple tuning adjustments 26 for controlling the RF delay.
Although such an aluminum-block comb-line bandpass filter 10 is able to provide low-loss delay at high power levels without the size and cost requirement of the bulky coaxial cable, the filter must be individually tuned using the tuning adjustments 26 of the tuner 24 and manually assembled, which are typically done by the supplier. Custom tuning and assembly adds extra manual labor to the cost of the filter 10. Specially, each of the tuning adjustments 26 shown has to be individually tuned by the supplier. Although the aluminum-block comb-line bandpass filter is cheaper than the coaxial cable, it is still fairly expensive since the production cost for each filter 10 costs approximately $45. Thus, the filter may be better than the coaxial cable, but it certainly has its own set of shortcomings, such as cost and labor.
There are other filters, such as a triple-mode ceramic delay filter and a tunable filter that has variable bandwidth and variable delay. All these other filters are similarly expensive and require specific tuning and manual assembly.
Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity and have not necessarily been drawn to scale. For example, the dimensions of some of the elements in the figures may be exaggerated relative to other elements to help improve understanding of various embodiments of the present invention. Also, common and well-understood elements that are useful or necessary in a commercially feasible embodiment are often not depicted in order to facilitate a less obstructed view of these various embodiments of the present invention.